The present invention relates to an apparatus and a method for cleaning a partial area of a substrate, in particular a photomask.
In many technical fields, and in particular in the semiconductor production field, cleaning of substrates may be challenging due to the required cleanliness of the cleaned substrates. One example of such a substrate, which requires high cleanliness, is a photomask, which is for example used in photolithography for the mass production of integrated circuit devices.
Any particles on the surfaces of such photomasks, in particular the patterned surfaces thereof may lead to errors in the substrates being imaged using the photomask. Therefore, regular cleaning of the photomasks is required, which, however may reduce the lifetime thereof. In order to protect certain surfaces of the photomasks from particle contamination, pellicles have been introduced. Pellicles are thin films or membranes, which are for example attached to a frame that is glued to one side of the photomask, such that the membrane acts as a cover preventing particles from reaching the covered surface. The pellicle is spaced far enough from the mask, so that moderate to small sized particles that may be present on the pellicle will be too far out of focus to influence imaging of the photomask. By using such pellicles, the cleaning cycles for photomasks may be reduced, thereby increasing the lifetime of the photomask and at the same time imaging results were improved.
However, such pellicle membranes have to be exchanged after a certain number of imaging cycles by removal of the pellicle frame, which is glued to the mask surface. After removal of the pellicle frame, some of the glue remains on the photomask, which has to be fully removed before a further pellicle membrane with frame may be attached to the photomask.
In the past, aggressive chemical cleaning solutions had to be used to remove the glue residuals from the surface of the photomask. For example, SPM has been employed over the past years to remove the typically polymeric glue residuals. Despite its good cleaning performances, SPM has created other problems such as haze generation. Further methods have been proposed to overcome the haze issues by for example using different chemistry which avoids haze generation. However, these chemistries often only allowed partial removal of the glue in acceptable time limits and full removal was difficult if at all achieved. For both the SPM and other chemistries, it was also not possible to only clean local areas of the photomask as the chemistry was not contained. Hence the chemistry was also applied to areas which did not need cleaning, thereby increasing the amount of chemistry needed and creating a danger of damaging those areas. Also dry removal methods such as laser ablation have been suggested to remove pellicle glue residuals, but these methods were very specific to a single use and difficult to control.